Tamper proof labels consisting of a transparent base foil, an intermediate layer and a cover foil are well-known. The intermediate layer is interposed between one surface of the base foil and a first adhesive layer. The cover foil carries a second adhesive layer and is applied to the other surface of the base foil. Preferably, the intermediate layer is a shiny metallic film created by vapor deposition. Adhesion between the base foil and the intermediate layer is decreased in preselected areas.
When the label is first applied, it appears to be of a uniform color throughout. As the label is removed, the intermediate layer with the adhesive layer clings to the base foil in the regions of high adhesion, while in the regions with the decreased adhesion the intermediate layer and the adhesive layer are detached from the base foil and cling to the surface to which the label is applied. The intermediate layer is thus cracked or split up, this splitting being clearly visible by changes in color or luster. The selected areas of low adhesion may be formed to create alphanumeric symbols or other graphics, such symbols or graphics becoming visible when the label is removed. The presence of such symbols or graphics therefore clearly indicates that a sealed package has been opened.
These well-known labels or seals have the disadvantage that, with some patience and skill, the base foil can be reapplied to the extent that the selected areas are reconstituted without any visible seam and therefore convey the impression that the label has never been removed. Further, the well-known labels can be removed at higher temperatures without splitting or cracking the intermediate layer. This facilitates unauthorized opening of the package, because no particular skill is required for reapplying the label. Finally, it is possible to paste a standard metallized foil or a foil of suitable color over the part of the original label remaining on the item, again yielding the impression that no tampering has occurred. An imprint on the top surface of the base foil could possibly be forged.